LESSONS  FROM  THE  NECTAR  BATS  OF  COLIMA,  MEXICO

MANILA, JUNE 8, 2008
(STAR) STAR SCIENCE By Raul K. Suarez, PH.D. - I began writing articles for the science section of The Philippine STAR last year when I was in Colima, Mexico, conducting research on nectar-feeding bats with my graduate student and Mexican scientists. This effort resulted in a discovery (recently published in the Journal of Experimental Biology, volume 311, pages 310-316, 2008) that I would like to share.

Once upon a time, only insects made a living by flying and feeding on the sugary nectar in flowers. But in the course of their evolution, hummingbirds and nectar-feeding bats, like insects, also evolved small bodies and the ability to hover while feeding on floral nectar. In the same way that fires require oxygen to burn, oxygen is required so that sugar can be used as a source of energy for the mechanical work performed by muscles. The rate of mechanical work during flight is so high in insects such as bees, as well as hummingbirds and nectar bats, that they “burn” sugar and consume oxygen at some of the highest rates known in the animal kingdom.

Our recent work on nectar bats is a logical extension of my research concerning the energy metabolism of nectar-feeding insects and hummingbirds. The high metabolic rates of hummingbirds and nectar bats at rest and during flight require high rates of dietary intake of sugary nectar. Just as in all animals (including humans), body weight is maintained when dietary energy intake equals daily energy expenditure. When energy intake exceeds expenditure, they gain weight. Most of this weight gain represents the build-up of body fat, synthesized from the sugar that they eat. Hummingbirds are well known for their remarkable ability to build up fat reserves. Individuals stopping to refuel during their seasonal migration between British Columbia and Mexico can gain 10 percent of total body weight per day. Migratory flight resumes when about 40 percent of their body weight consists of fat. In contrast, the nectar bats that we studied do not migrate and do not accumulate much fat.

When hummingbirds wake up in the morning after fasting overnight, they use fat as their main fuel for flight. As they begin to feed on sugar, their metabolism shifts such that about 95 percent of energy requirements are met using recently eaten sugar. More than a year ago, we wondered whether nectar bats evolved such that they became hummingbird-like in their metabolism. Conventional wisdom could have led to the outright dismissal of this idea: sugar drinks are able to directly fuel only 25-30 percent of the requirements of muscles during exercise in humans.

In Colima, we caught the bats in banana plantations in the middle of the night by setting up nets close to banana flowers. When the bats came flying in to feed on the banana flowers’ nectar, they got tangled in the nets. We transported them to holding cages in a rented house and performed our experiments by letting them fly in a big tent in the back yard. The bats slept in the day and were active at night, and we usually had to wait until past midnight before they began to hover to feed from our artificial flower. While very much sleep-deprived, I did not appreciate it when the neighbor’s dog would bark, inhibiting our bats from hovering and feeding. I resisted the temptation to poison the neighbor’s dog, and we eventually succeeded in measuring the rates at which the bats consumed oxygen and produced carbon dioxide as they fed on sugar solutions while hovering. The solutions contained either sugar made from beets or from sugar cane. Because of peculiarities of the process of photosynthesis, beet and cane sugars have different carbon stable isotope compositions. So, while we normally fed the bats solutions of beet sugar, we used cane sugar in experiments and sampled bat breath to analyze changes in its carbon stable isotope composition as the bats hovered to feed. These data allowed the calculation of the contribution made by dietary sugar to metabolism during flight. As in the case of hummingbirds, bats began flying (after their daytime fast) using mainly fat as their fuel. But as the bats fed, we found that recently ingested sugar provided close to 80 percent of the fuel needs of their exercising muscles.

We therefore discovered the first mammal known to fuel exercise mainly through the direct use of ingested sugar. The underlying mechanisms are currently the subject of further research in my laboratory. Why should American taxpayers pay for studies on Mexican bats? This is a case of convergent evolution, involving mammals that became like hummingbirds and nectar-feeding, flying insects. From an ecological perspective, certain flowering plants have become dependent on nectar bats for pollination. Therefore, changes in the abundance and distribution of one likely affect the other. Lastly, the consumption of sugary soft drinks has been linked to childhood obesity. Unlike children, the nectar bats of Colima, Mexico, do not become obese despite eating mostly sugar, staying up all night and sleeping all day. We have much to learn from them and about their role in the natural world.

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Raul Kamantigue Suarez is a professor in the Department of Ecology, Evolution and Marine Biology of the University of California, Santa Barbara, California and an editor of the Journal of Experimental Biology, Cambridge, UK. E-mail him at suarez@lifesci.ucsb.edu.


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